1. Field of the Invention
This invention generally relates to semiconductor radiation detectors and methods of making same and, in particular, to semiconductor detectors designed to detect neutrons of various energy ranges, methods of making same and measuring wands and detector modules utilizing such detectors.
2. Background Art
The following references are referenced herein:    1. D. S. McGregor, J. T. Lindsay, C. C. Brannon and R. W. Olsen, “Semi-insulating Bulk GaAs Thermal Neutron Imaging Arrays,” IEEE Trans. Nucl. Sci., NS-43 (1996) p. 1357.    2. A. Rose, “Sputtered Boron Films on Silicon Surface Barrier Detectors,” Nucl. Instr. and Meth., 52 (1967) p. 166.    3. B. Feigl and H. Rauch, “Der Gd-Neutronenzahler,” Nucl. Instr. and Meth., 61 (1968) p. 349.    4. A. Mireshghi, G. Cho, J. S. Drewery, W. S. Hong, T. Jing, H. Lee, S. N. Kaplan and V. Perez-Mendez, “High Efficiency Neutron Sensitive Amorphous Silicon Pixel Detectors,” IEEE Trans. Nucl. Sci., NS-41 (1994) p. 915.    5. F. Foulon, P. Bergonzo, A. Brambilla, C. Jany, B. Guizard and R. D. Marshall, “Neutron Detectors Made from Chemically Vapour Deposited Semiconductors,” Proc. MRS, 487 (1998) p. 591.    6. A. R. Dulloo, F. H. Ruddy, and J. G. Seidel, “Radiation Response Testing of Silicon Carbide Semiconductor Neutron Detectors for Monitoring Thermal Neutron Flux,” Report 97-9TK1-NUSIC-R1, Westinghouse STC, Pittsburgh, Pa. (Nov. 18, 1997).    7. G. F. Knoll, Radiation Detection and Measurement, 3rd Ed. (Wiley, New York, 2000).    8. D. I. Garber and R. R. Kinsey, BNL 325: Neutron Cross Sections, 3rd Ed., Vol. 2, Curves (Brookhaven National Laboratory, Upton, 1976).    9. V. McLane, C. L. Dunford and P. F. Rose, Neutron Cross Sections, Vol. 2 (Academic Press, San Diego, 1988).    10. D. S. McGregor, R. T. Klann, H. K. Gersch, and Y-H. Yang, “Thin-Film-Coated Bulk GaAs Detectors for Thermal and Fast Neutron Measurements,” Nuclear Instruments and Methods, A466 (2001) pp. 126-141.    11. D. S. McGregor, M. D. Hammig, H. K. Gersch, Y-H Yang, and R. T. Klann, “Design Considerations for Thin Film Coated Semiconductor Thermal Neutron Detectors, Part I: Basics Regarding Alpha Particle Emitting Neutron Reactive Films,” Nuclear Instruments and Methods, A500 (2003) pp. 272-308.    12. J. K. Shultis and D. S. McGregor, “Calculation of Ion Energy-Deposition Spectra in Silicon, Lithium-Fluoride, Boron, and Boron Carbide,” Report 299, Engineering Experiment Station, Kansas State University, Manhattan, Kans., 2004, http://ww2.mne.ksu.edu/˜jks/papers/EESrpt299.pdf (referenced on Sep. 20, 2005).    13. J. K. Shultis and D. S. McGregor, “Efficiencies of Coated and Perforated Semiconductor Neutron Detectors,” IEEE Trans. Nuclear Science, NS-53 (2006) pp. 1659-1665.    14. D. S. McGregor, R. T. Klann, H. K. Gersch, E. Ariesanti, J. D. Sanders, and B. VanDerElzen, “New Surface Morphology for Low Stress Thin-Film-Coated Thermal Neutron Detectors,” IEEE Trans. Nuclear Science, 49 (2002) pg. 1999-2004.
Semiconductor detectors coated with neutron reactive materials offer an alternative approach to scintillator-based neutron imaging devices for neutron radiography (normally scintillating screens coupled to photographic film or to other photorecording devices). Neutron reactive film-coated devices investigated in previous works include Si, SiC, GaAs, and diamond detectors, all of which have advantages and disadvantages [1-6].
The converter films attached to semiconductor devices most often used for neutron detection utilize either the 6Li(n,α)3H reaction or the 10B(n,α)7Li reaction. Due to low chemical reactivity, the most common materials used are pure 10B and 6LiF. Neutron reactive films based on the 157Gd(n,γ)158Gd reaction show a higher neutron absorption efficiency than 10B(n,α)7Li and 6Li(n,α)3H-based films, however the combined emission of low energy gamma rays and conversion electrons from 157Gd(n,γ)158Gd reactions make neutron-induced events difficult to discriminate from background gamma-ray events. As a result, Gd-based films are less attractive for devices where background gamma ray contamination is a problem. Alternatively, the particle energies emitted from the 6Li(n,α)3H and the 10B(n,α)7Li reactions are relatively large and produce signals easily discernable from background gamma ray noise.
Expected Efficiency of Conventional 10B and 6Li Coated Planar Detectors
The 10B(m,α)7Li reaction leads to the following reaction products [7]:
                                                       10                    ⁢          B                +                                                           0              1                        ⁢            n                    ⟶                      {                                                                                                                                                                                                                 7                                                    ⁢                          Li                                                ⁡                                                  (                                                      at                            ⁢                                                                                                                  ⁢                            1.015                            ⁢                                                                                                                  ⁢                            MeV                                                    )                                                                    +                                              α                        ⁡                                                  (                                                      at                            ⁢                                                                                                                  ⁢                            1.777                            ⁢                                                                                                                  ⁢                            MeV                                                    )                                                                                      ,                                                                                        2.792                    ⁢                                                                                  ⁢                                          MeV                      ⁡                                              (                                                  to                          ⁢                                                                                                          ⁢                          ground                          ⁢                                                                                                          ⁢                          state                                                )                                                                                                                                                                                                                                                                                               7                                                    ⁢                          Li                                                *                                                  (                                                      at                            ⁢                                                                                                                  ⁢                            0.840                            ⁢                                                                                                                  ⁢                            MeV                                                    )                                                                    +                                              α                        ⁡                                                  (                                                      at                            ⁢                                                                                                                  ⁢                            1.470                            ⁢                                                                                                                  ⁢                            MeV                                                    )                                                                                      ,                                                                                        2.310                    ⁢                                                                                  ⁢                                          MeV                      ⁡                                              (                                                  1                          ⁢                                                                                   st                                                    ⁢                                                                                                          ⁢                          excited                          ⁢                                                                                                          ⁢                          state                                                )                                                                                                                                                    Reaction        ⁢                                  ⁢        Q        ⁢                  -                ⁢        Value            which are released in opposite directions when thermal neutrons (0.0259 eV) are absorbed by 10B. After absorption, 94% of the reactions leave the 7Li ion in its first excited state, which rapidly de-excites to the ground state (˜10−13 seconds) by releasing a 480 keV gamma ray. The remaining 6% of the reactions result in the 7Li ion dropping directly to its ground state. The microscopic thermal neutron absorption cross section is 3840 barns. Additionally, the microscopic thermal neutron absorption cross section decreases with increasing neutron energy, with a dependence proportional to the inverse of the neutron velocity (1/v) over much of the energy range [8,9].
The 6Li(n,α)3H reaction leads to the following products:
                             6            ⁢      Li        +                                       0          1                ⁢        n            ⟶                                                 3                    ⁢          H                ⁡                  (                      at            ⁢                                                  ⁢            2.73            ⁢                                                  ⁢            MeV                    )                      +          α      ⁡              (                  at          ⁢                                          ⁢          2.05          ⁢                                          ⁢          MeV                )              ,            Reaction      ⁢                          ⁢      Q      ⁢              -            ⁢      Value              4.78      ⁢                          ⁢      MeV      which again are oppositely directed if the neutron energy is sufficiently small. The microscopic thermal neutron (0.0259 eV) absorption cross section is 940 barns. The thermal neutron absorption cross section also demonstrates a 1/v dependence, except at a salient resonance above 100 keV, in which the absorption cross section surpasses that of 10B for energies between approximately 150 keV to 300 keV [8,9]. Additional resonances characteristic to either isotope cause the absorption cross section to surpass one or the other as the neutron energy increases. Due to its higher absorption cross section, the 10B(n, α)7Li reaction leads to a generally higher reaction probability than the 6Li(n,α)3H reaction for neutron energies below 100 keV. However, the higher energy reaction products emitted from the 6Li(n,α)3H reaction lead to greater ease of detection than the particles emitted from the 10B(n,α)7Li reaction.
The term “effective range” (denoted L) is the distance through which a particle may travel within the neutron reactive film before its energy decreases below the set minimum detectable threshold, or rather, before its energy decreases below the electronic lower level discriminator (LLD) setting. The term does not take into account additional energy losses from contact “dead regions”. The neutron reaction products released do not have equal masses, and therefore do not have equal energies or effective ranges. Neutrons may interact anywhere within the reactive film, and the reaction products lose energy as they move through the neutron reactive film. Reaction product self-absorption reduces the energy transferred to the semiconductor detector, and ultimately limits the maximum film thickness that can be deposited over the semiconductor device. The measured voltage signal is directly proportional to the number of electron-hole pairs excited within the semiconductor. Reaction products that deposit most or all of their energy in the detector will produce much larger voltage signals than those reaction products that lose most of their energy before reaching the detector.
The energy absorbed in the detector is simply the original particle energy minus the combined energy lost in the boron film and the detector contact during transit. At any reaction location within the reactive film, a reduced energy will be retained by either particle that should enter the detector, being the maximum possible if the trajectory is orthogonal to the device contact. Hence, if the interaction occurs in the 10B film at a distance of 0.5 microns away from the detector, the maximum energy retained by the 7Li ion when it enters the detector will be 430 keV, and the maximum energy retained by the alpha particle will be 1150 keV [10,11]. For the same interaction distance of 0.5 microns from the detector, the energy retained by the particle when it reaches the detector decreases as the angle increases from orthogonal (>0°). Given a predetermined minimum detection threshold (or LLD setting), the effective range (L) for either particle can be determined. For instance, an LLD setting of 300 keV yields LLD as 0.810 microns and La as 2.648 microns [10,11]. Similar conditions exist for 6LiF and 6Li films.
A commonly used geometry involves the use of a planar semiconductor detector over which a neutron reactive film has been deposited (see FIG. 1). Assuming that the neutron beam is perpendicular to the detector front contact, the sensitivity contribution for a reaction product species can be found by integrating the product of the neutron interaction probability and the fractional solid angle, defined by the reaction product effective ranges subtending the device interface [1,1], which yields:
                                                        S              p                        ⁡                          (                              D                F                            )                                =                                    0.5              ⁢                              F                p                            ⁢                              {                                                                            (                                              1                        +                                                  1                                                                                    Σ                              F                                                        ⁢                            L                                                                                              )                                        ⁢                                          (                                              1                        -                                                  ⅇ                                                                                    -                                                              Σ                                F                                                                                      ⁢                                                          D                              F                                                                                                                          )                                                        -                                                            D                      F                                        L                                                  }                            ⁢                                                          ⁢              for              ⁢                                                          ⁢              D                        ≤            L                          ,                                  ⁢        and                            (                  1          ⁢          A                )                                                                    S              p                        ⁡                          (                              D                F                            )                                =                                    0.5              ⁢                              F                p                            ⁢                              ⅇ                                  -                                                            Σ                      F                                        ⁡                                          (                                                                        D                          F                                                -                        L                                            )                                                                                  ⁢                              {                                                                            (                                              1                        +                                                  1                                                                                    Σ                              F                                                        ⁢                            L                                                                                              )                                        ⁢                                          (                                              1                        -                                                  ⅇ                                                                                    -                                                              Σ                                F                                                                                      ⁢                            L                                                                                              )                                                        -                  1                                }                            ⁢                                                          ⁢              for              ⁢                                                          ⁢                              D                F                                      >            L                          ,                            (                  1          ⁢          B                )            where ΣF is the macroscopic neutron absorption cross section, DF is the film thickness, and Fp is the branching ratio of the reaction product emissions. The total sensitivity accordingly can be found by adding all of the reaction product sensitivities
                                                        S              ⁡                              (                                  D                  F                                )                                      ⁢                          ❘              Total                                =                                    ∑                              p                =                1                            N                        ⁢                                          S                p                            ⁡                              (                                  D                  F                                )                                                    ,                            (        2        )            where N is the number of different reaction product emissions. In the case of 10B-based films, N equals 4. Notice from equation 1B that the value of SP reduces as DF becomes larger than the value of L. As a result of this, there will be an optimum neutron reactive film thickness for front-irradiated detectors [1,1]. Since the minimum particle detection threshold determines the effective range (L), the optimum film thickness is also a function of the LLD setting. With the LLD set at 300 keV, the maximum achievable thermal neutron detection efficiency is 3.95%. The thermal neutron detection efficiency can be increased to 4.8% by lowering the LLD setting, but only at the expense of accepting more system noise and gamma-ray background interference [1,10,11]. Similar cases exist for 6LiF and pure 6Li films. Using an LLD setting of 300 keV, obverse detector irradiation yields maximum thermal neutron detection efficiencies of 4.3% for 6LiF-coated devices and 11.6% for pure 6Li-coated devices [1,1].Perforated 10B and 6Li Coated Detectors Using Either Circular Holes or Straight Channels Irradiated with a Neutron Beam Perpendicular to the Detector
To increase the detection efficiency of a semiconductor neutron detector, a lattice of circular holes, as depicted in FIG. 2, or straight channels, as depicted in FIG. 3, can be etched into the semiconductor material and subsequently filled with 10B, 6LiF or some other neutron reactive film. The circular hole design was analyzed for 10B and 6LiF materials, along with numerous overcoat or cap layer thicknesses. It was found that interactions in the perforations backfilled with neutron reactive material dominated the detector efficiency and that tremendous efficiency increases could be realized [12,13]. FIGS. 4 and 5 show the expected efficiencies for 6LiF-coated and backfilled devices for obverse irradiation as a function of overlayer or ‘cap’ thickness and cell pitch or dimension. The perforation depths in FIGS. 4 and 5 are 300 microns. Shown in FIG. 4 are efficiency curves for 6LiF backfilled detectors with hole diameter equal to 60% of the pitch between the holes. For instance, a pitch or cell dimension of 50 microns would have hole diameters of 30 microns. Shown in FIG. 5 are efficiency curves for 6LiF backfilled detectors with channel widths equal to 50% of the pitch between the holes. For instance, a pitch or cell dimension of 50 microns would have channel widths of 25 microns. Both FIGS. 4 and 5 show the dependence of neutron detection efficiency as a function of the 6LiF layer thickness added on top of the detector. From these results it is seen that efficiencies exceeding 30% can be attained and that an overlayer or a cap layer thickness of 20 microns yields near optimum results [1,3]. However, the calculations in the literature [12,13] do not take into account the adverse effect that angular trajectories from neutrons have on the efficiency as they intersect the detector. To remedy this adverse effect is one of the main objects of at least one embodiment of the invention herein.
Problems of Streaming and Non-Uniform Neutron Detection Response
In the design of perforated detectors as described in U.S. Pat. No. 6,545,281 B1 and U.S. Pat. No. 7,164,138 B2 there exists an inherent problem. The problem is a result of the repeated lattice structure substrate material composed of circular holes filled with neutron sensitive material as shown in FIG. 2 (referred to as ‘rods’), or straight channel or channels filled with neutron sensitive materials, as depicted in FIG. 3, between which are ‘fins’ of semiconductor material. FIG. 6 shows a top view of a perforated semiconductor substrate 1 with channels 9 separated by fins 11 of semiconductor material. It is to be understood that the same reference numerals are used throughout the drawing figures to designate the same or similar structures unless otherwise indicated. For example, reference numeral “1” refers to a semiconductor substrate throughout the drawing figures.
Shown in FIG. 7, a cross sectional view, is the circumstance in which a perforated detector 14 is irradiated with a neutron source 10. The neutron streaming paths 15 in FIG. 7 show trajectories in which neutrons can pass through the device without intersecting the neutron reactive material 3 in the channel perforations 9. The streaming paths occur within specific angle ranges, hence if the detector is turned at these angles with respect to the neutron source, then there will be reductions in the observed efficiency. From prior art, as shown in FIG. 8, two perforated detectors 14 can be faced into each other to increase the neutron detection efficiency. However, this case works provided that the detector is aligned perpendicular to the neutron 4 paths. It is far more common that neutrons intersect the detectors at non-perpendicular angles.
Shown in FIG. 9, a cross sectional view, is the circumstance in which two perforated detectors 14 are facing each other, stacked, with a neutron reactive material 3 in the perforations 9 and between the detectors 14. The neutrons interacting in the neutron reactive material contained within the cylindrical holes or straight channels 9 will cause the neutron reactive material to eject energetic charged particle reaction products that afterwards can enter and interact within the adjacent semiconductor material 11. These charged particle emissions are then detected in the semiconductor material which constitutes the detection of a neutron. However, as shown in FIG. 9, neutrons from the source 10 can “stream” 15 through regions in the detectors which will reduce the detection probability. Hence, there are again particular angles at which neutrons can arrive at the detector which will reduce detection, resulting in an undesirable circumstance.
FIG. 10 shows the normalized angular probability of an interaction occurring in two perforated straight channel structures, both filled with 10B, facing each other in a sandwich as depicted in FIG. 9. FIG. 10 shows the results for different channel depths, where the depth of the channels is designated as ‘L’ in FIG. 10. The channel width is 6 microns and the semiconductor fin thickness is also 6 microns. The cap layer atop the detector is 1 micron. It becomes apparent that the absorption probability is a strong function of the angle of incidence, showing a clear depression at angles between 5 to 15 degrees. If the boron-filled channels are allowed to overlap such that the channels are 6 micron wide and the semiconductor fins are 4 microns wide, the problem still remains, as shown in FIG. 11.
Angular Efficiencies of Neutron Detectors with Circular Hole or Straight Channel Perforations
The efficiency of a perforated neutron detector is a strong function of the incident neutron's direction. If one considers the axis of a perforated detector to be the polar axis of neutron incidence then it has been shown that as the angle of incidence increases from 0 to 90 degrees the detector efficiency falls off as a function of the cosine of the polar angle. This is an effect of the detector's solid angle, as seen by the source, being reduced to virtually nothing [1,1]. As a result, regardless of the design, the efficiency of the detector will drop as the polar angle of incidence increases.
Fluctuations in the efficiency resulting from changes in the azimuthal angle can be almost eliminated by appropriately designing the absorption region of the detector. Initial designs of perforated detectors considered perforations of cylindrical holes and channels into a silicon substrate. Both 10B and 6LiF were initially considered for the neutron sensitive material. The coordinate system used to describe the neutron efficiency and streaming effects in circular hole (or rod) perforated detectors is shown in FIG. 12 and the coordinate system used to describe the neutron efficiency and streaming effects in straight channel perforated detectors is shown in FIG. 13.
Perforated rod type detectors were modeled using a combination of the MCNP transport code and a specialized ion-transport code. Results of these simulations are presented in FIGS. 14-17 for varying material and perforation depths. One notices that at incident neutron azimuthal angles of multiples of 45 degrees that the detection efficiency drops dramatically, more so for even multiples of 45 degrees. This drop in efficiency is a result of neutrons streaming through regions of the detector and seeing less or no absorber material. FIG. 18 illustrates the concern. At azimuthal angles of 45 degrees the neutron has a small slot in between the rods that it may stream through without interacting. At 90 degrees the slot is much greater explaining the much greater drop in detection efficiency. Though the characteristics of the angular efficiencies are less than desirable, it should be pointed out that normal incidence efficiencies are predicted up to 18% for both 10B and 6LiF.
The drops in detection efficiency can be mitigated to some extent by using channels instead of rods. Channel perforated detectors were modeled using specialized transport codes written specifically for this application. FIGS. 19 and 20 represent plotted output from these codes and demonstrate the lessening of the azimuthal fluctuations. One notices in these cases that the efficiency changes much less with the azimuthal angle of incidence of the neutrons, and the reason is demonstrated in FIG. 13. The only streaming paths present occur at multiples of 90 degrees thereby flattening out the azimuthal response for all other angles. The adverse effect of efficiency non-uniformity as a function of neutron incidence angle is addressed by at least one embodiment of the invention herein.
Problems with Fragility
The thin fins of semiconductor material separating the linear channels in designs of prior art discussed in U.S. Pat. No. 7,164,138 B2 have proven to be fragile. The semiconductor material of the fins ranges from only a few microns wide to tens of microns wide, which have been found to be mechanically fragile. As a result, during the fabrication processes to manufacture the device, a large percentage of the fins crack and render that section of the detector insensitive. The adverse effect of fin fragility is addressed by at least one embodiment of the invention.
Problems with Fabrication
As previously described, prior art methods for designing and fabricating perforated diodes describe a basic concept for improving thermal neutron detection efficiency over simple planar-coated diodes. The idea, as described in U.S. Pat. No. 6,545,281 B1 and U.S. Pat. No. 7,164,138 B2, involves increasing the probability of capturing reaction products from thermal neutron interactions in neutron reactive films, primarily based upon Li and B based elements and compounds that contain some amount of 6Li and/or 10B isotopes. As previously described and illustrated herein, in these devices, circular holes or straight parallel channels are etched into a semiconductor diode structure which are subsequently backfilled with Li or B based materials. These designs work and are effective. However, the relatively simple constructions as described by the prior art are not optimized. Further, the construction techniques described in prior art are relatively difficult to make and result in relatively inferior performing detectors with relatively low reliability.
FIG. 21 is a side cross sectional view of a prior art detector at its basic level as described in U.S. Pat. No. 6,545,281 B1 and U.S. Pat. No. 7,164,138 B2. The basic device or detector 14 includes a semiconductor substrate 1 which has a back side conductive contact 2 applied to the semiconductor substrate 1. A junction is built at the top of the semiconductor substrate 1 by the introduction of impurity dopants into the substrate to form a rectifying junction 21 or by forming a metal-semiconductor Schottky junction. In either case, the entire top surface of the semiconductor substrate 1 forms the rectifying junction 21. Afterwards, the top of the semiconductor substance 1 has perforations 9 etched vertically down which are either circular holes or straight channels, as described by the prior art. The etched perforations 9 are cut through the rectifying junction, as shown in FIG. 21. Metal contacts 2 are then applied uniformly over the bottom surface of the substrate and over the top surface of the junction 21. Lastly, a neutron reactive material 3, such a B or LiF is coated over the device 14 to fill the perforations 9.
It has been learned that etching through the rectifying junction damages it and reduces performance. FIG. 22 shows the difference between the reverse current of a pn junction diode before the perforations are etched and after the perforations were etched. It is clearly shown that the leakage increased by more than a factor of 1000. The adverse effect of high leakage currents caused from etching perforations through the rectifying junction is addressed by at least one embodiment of the invention.
As previously described, 6LiF is a prospective neutron reactive material for coated neutron detectors. Neutrons interacting in the 6Li material are absorbed and immediately cause the Li atom to fission into a 4He ion and a triton (3H) with energies of 2.05 MeV and 2.73 MeV, respectively.
6LiF is a semi-stable material the can be produced easily with the following reaction process steps. First, enriched Li metal is placed into purified and de-ionized (DI) water, which reacts for form LiOH. The LiOH solution is saturated with slivers of Li metal until the reaction visibly stops. Afterwards, a dilute solution of HF and DI water is titrated or dripped into the LiOH solution to reaction and form LiF and water. Hydrogen gas is given off as a by product. The process is continued until the pH of the solution is between 6.8 and 7.2. The solution of LiF and water is allowed to stand until the LiF falls to the bottom of the container. The water on top is poured out into filter paper to catch any residual LiF still suspended in the water. The slurry of LiF paste is then poured into a petri dish and dried in a oven at 50° C. for 12 hours to form a dried powder in the petri dish. The LiF is then ready for use as a filling material for the detector perforations.
As also previously described, 10B is also a prospective neutron reactive material for coated neutron detectors and can be purchased from commercial vendors. Neutrons interacting in the 10B material are absorbed and immediately cause the B atom to fission into a 4He ion and a 7Li ion with energies (94% branching) of 1.47 MeV and 0.84 MeV, or (6% branching) of 1.78 MeV and 1.05 MeV, respectively.
Prior attempts to fill detector perforations with either LiF or B have thus far included physical vapor deposition with thermal and e-beam evaporation (FIG. 32), powder filling with ultra-sonics (FIG. 33), thermal melting (FIG. 34), and plasma sputtering. These methods are described in U.S. Pat. No. 6,545,281 B1 and U.S. Pat. No. 7,164,138 B2. All of these methods have shortcomings and are not optimal in filling of the perforations in semiconductor diode detector structures.
Physical vapor deposition as suggested in the prior art of U.S. Pat. No. 6,545,281 B1 and U.S. Pat. No. 7,164,138 B2 was successfully used to fill the so described perforated diodes. This technique requires that either the B or LiF source material be placed in a thermally tolerant holder, such as a graphite crucible or a tungsten filament “boat.” The holder is filled with material and is then placed in a vacuum chamber. The perforated structures are then placed inside the vacuum chamber with the perforations of the structures facing directly at the material-filled holder. A vacuum is pulled on the deposition system such that the mean free path of a free particle ranges above 50 cm, preferably greater than 1 meter. The LiF-filled or B-filled holder is heated to high temperatures such that the source material melts and evaporates. This is usually achieved by directing an energetic electron beam into the LiF-filled or B-filled crucible, or by passing a high current through the LiF-filled or B-filled filament. The evaporated LiF molecules or B atoms stream in straight paths from the holder, some of which strike the perforated diode structure. The LiF or B material can enter into the holes and partially fill them. Further, the LiF or B material attaches to the surface around the holes.
This method has problems, for it only works for very shallow holes. Deeper holes become plugged up. FIG. 32 shows a cross section of a shallow LiF-filled hole that has been partially filled by thermal evaporation. The hole is becoming blocked or plugged from LiF closing the hole off at the entrance. As a result, the hole can neither be completely filled, nor can deep holes be filled using this technique. This result has been observed for physical vapor deposition of B in perforations [1,4]. Sputtering, another vacuum deposition method, has the same above-noted problems as physical vapor deposition.
Powder filling as suggested in prior art U.S. Pat. No. 6,545,281 B1 and U.S. Pat. No. 7,164,138 B2 has also been used. As described therein, either B or LiF powder is spread over a perforated surface and spread powder is either ultrasonically shaken into the holes or is physically pressed into the holes. Ultrasonic vibration does assist with the B powder filling, however it does not work well for LiF powder filling. Although the method does work to partially fill the holes with B powder and for LiF powder as shown in FIG. 33, the void space left behind in the holes can amount to over 30% unused space. Furthermore, the techniques are labor intensive and unsuitable for mass production.
Thermal melting as suggested in prior art U.S. Pat. No. 6,545,281 B1 and U.S. Pat. No. 7,164,138 B2 has been used for LiF, as shown in FIG. 34. In such a case, the LiF powder is spread over the semiconductor perforated structure. Thermal infra-red lamps are used to melt the LiF into the holes. While this method completely fills the holes, it also ruins the diode properties. Both Li and B are dopant impurities in Si, and thermal melting causes the Li or B to ruin the rectifying junctions. Further, the melting point of B exceeds 1200° C. in vacuum, making the method impractical for serious processing in semiconductor diodes.
The following U.S. patents are also related to the present invention: RE35,908 E; 5,629,523 A; 5,880,471 A; 5,940,460 A; and 6,479,826 B1.